[NETFILTER]: conntrack: introduce connection mark event

This patch introduces the mark event. ctnetlink can use this to know if
the mark needs to be dumped.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>